Project Summary/Abstract: Dominantly inherited mutations in LRRK2 and ?-synuclein are the most common genetic cause of Parkinson's disease (PD), and ?-synuclein aggregates are a major component of Lewy bodies, which are a key neuropathological hallmark of PD. Earlier studies found that the LRRK2 G2019S mutation enhanced its kinase activity in vitro, suggesting that LRRK2 mutations may cause PD via a toxic gain of function mechanism. Surprisingly, overproduction of mutant LRRK2 including G2019S even at very high levels (>10-fold) failed to cause neurodegeneration in mice, whereas inactivation of LRRK2 results in PD-like changes, such as age- dependent impairment of autophagy and protein degradation pathways, striking accumulation and aggregation of ?-synuclein, and increases of apoptosis, inflammation and oxidative damage in the kidney where LRRK2 is normally most abundant. Our recent generation and characterization of double knockout (DKO) mice lacking LRRK2 and its functional homologue LRRK1 showed that they develop similar changes in the brain, including age-dependent, selective loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) and noradrenergic neurons in the locus coeruleus, accompanied with autophagy impairment, ?-synuclein accumulation, and increases of apoptosis and inflammation. These findings suggest an interesting genetic interaction between LRRK2 and ?-synuclein, and highlight a critical role of LRRK in DA neuron survival and autophagy-lysosomal pathways during aging. In this application, we propose two Specific Aims to investigate the pathogenic mechanisms underlying LRRK2 mutations, and to explore the genetic interaction between LRRK2 and ?-synuclein. In Aim 1, we propose to develop LRRK conditional DKO mice to determine the consequences of loss of LRRK function in the SNpc or the striatum during aging, and will also generate inducible human LRRK2 G2019S transgenic mice in the LRRK2-null background to determine whether in the absence of endogenous LRRK2, overproduction of the human G2019S mutant results in DA neurodegeneration. In Aim 2, we propose to use genetic approaches to explore the genetic interaction between LRRK2 and ?-synuclein and determine whether loss of LRRK results in age-dependent neurodegeneration via ?-synuclein accumulation. The completion of the proposed studies will provide new insight into the pathogenic mechanism underlying LRRK2 mutations as well as the relationship between LRRK2 and ?-synuclein.